1. Field of the Invention
The present invention relates to a mobile terminal position calculation method, positioning system, and position calculation apparatus for measuring the position of a mobile terminal, using radio signals.
2. Description of Related Art
In a cellular telephony system, a method of calculating the present position of a mobile terminal, using signals transmitted from base stations to the mobile terminal has been proposed. For example, such a method is described in Patent Document Cited 1, JP-A No. 181242/1995.
In the method of measuring the present position of a mobile terminal, described in Patent Document Cited 1 (FIGS. 14 and 15), first, in an action of measuring differences between propagation distances 110, measurements are taken of differences between propagation distances of signals from the mobile terminal to multiple base stations. Concretely, a mobile-terminal 104 receives signals transmitted from cellular telephony base stations 101, 102, and 103. At this time, the mobile terminal 104 records the times at which the respective signals from the base stations have arrived at the mobile terminal. By calculating differences between the signal arrival times, time differences of signal propagation on the channels from the base stations to the mobile terminal T1−T2 and T3−T2 are calculated. Finally, the thus calculated propagation time differences are multiplied by light velocity and, thereby, differences between the propagation distances of the signals from the base stations to the mobile terminal D1−D2=c (T1−T2) and D3−D2=c(T3−T2) are calculated.
Next, in an action of calculating MT position by trilateration 111, the position of the mobile terminal is calculated, using the principle of trilateration. Concretely, a relation (equation) given in mathematical expression 1 between propagation distance differences D1−D2 and D3−D2 measured in the action of measuring differences between propagation distances 110 and the position of the mobile terminal (X, Y) is solved to obtain unknown values (X, Y) of the mobile terminal position. In mathematical expression 1, c represents light velocity and DN (N=1, 2, 3) represents distance from the mobile terminal to base station N.c(T1−T2)=D1−D2 c(T3−T2)=D3−D2 DN={(X−XN)2+(Y−YN)2}1/2(N=1, 2, 3)  [Mathematical expression 1]
Finally, in an action of outputting MT position 112, the mobile terminal position obtained by solving the equation of mathematical expression 1 is output as the result of position calculation.
Also, a method of calculating the present position of a mobile terminal, using signals transmitted from the mobile terminal and received by a plurality of base stations has been offered; for example, such a method using signals transmitted on a wireless LAN, disclosed in JP-B No. 260772/2002 corresponding to U.S. application Ser. No. 10/463,608, a prior application made by one of the present inventors.
In the foregoing method of prior art, as shown in FIGS. 14 and 15, firstly, the position of a mobile terminal cannot be obtained according to conditions of measurement. For example, if |c(T1−T2)|>D12 (where D12 is distance between base station 1 and base station 2) due to the influence of measurement errors of propagation time differences, the condition of forming a triangle among the mobile terminal, base station 1, and base station 2 is not satisfied and the equation of mathematical expression 1 does not give a real root. Especially, when the mobile terminal is in close proximity to a base station, the above condition |c(T1−T2)|>D12 is met by even a small degree of measurement errors and, therefore, the probability that the equation of mathematical expression 1 does not give a real root increases. In such cases, the position of the mobile terminal cannot be obtained by the method of prior art.
In general, the equation of mathematical expression 1 gives a plurality of position solutions. If the mobile terminal is in the proximity of a base station, there is the plurality of position solutions to the equation of mathematical expression 1 near the base station. Consequently, it is impossible to select a proper solution from the plurality of position solutions of the mobile terminal. Thus, in this case also, the position of the mobile terminal cannot be obtained.
As mentioned above, when the mobile terminal is positioned in the proximity of a base station, its position cannot be obtained, and an area where a position information service is offered is narrowed and convenience is lowered.
Secondly, a rise in the cost of the position information service may be induced. Because the equation of mathematical expression 1 is difficult to solve analytically, a method of obtaining a solution by repetitive arithmetic operations is generally taken. This method involves large amounts of arithmetic operations and it may be necessary to repeat arithmetic operations hundreds of thousand of times according to conditions, and a high load is applied to an arithmetic unit which executes position calculation. When making use of applications which tolerate position measurement errors within a few hundred meters, for example, weather forecasts and information services about road traffic congestion and the like, the arithmetic unit must have performance enough to cope with excessive loads and this causes a rise in the cost of mobile terminals and the cost of the position information service.